Sheet registration systems are well known in the art and used to control, correct, and change the orientation and/or position of a sheet. Sheet registration systems use nips to drive paper along a feed path. The nips consist of a driven wheel and an idler wheel. The nips are mounted with bearings on a shaft so that the nips can rotate and translate. An angular velocity is imported on each of the driven wheels with a motor, which may be connected directly to the driven wheels or may be connected through a transmission (e.g., a timing belt). The motor may be a stepper motor or a DC servo motor with encoder feedback from an encoder mounted on either the motor shaft, driven wheel shaft, or idler shaft. Only one encoder is necessary for each set of nips to control the angular velocity of the driven wheel. The other two encoders may or may not provide additional functionality, but could be removed to save costs.
The nips are mounted such that they can move in the y-direction. In the teachings of U.S. Pat. No. 5,094, 442, the inboard and outboard motors, nips, etc. are all mounted inside a carriage that can move in the y-direction. U.S. Pat. Nos. 6,533,268 and 6,585,458 disclose a different mechanism to allow a y-direction motor with an appropriate actuator. According to this method the sheet can move in three degrees of freedom, i.e. x-direction (or process), y-direction (or lateral), and angular (or skew). The average of the velocities of each of the nips impart the process velocity, the differences in the nip velocities impart the angular velocity, and the y-direction actuator imparts a lateral motion.
U.S. Pat. No. 7,422,211 provides an example of a method for closed loop feedback for skew and lateral registration. The method uses edge sensors to measure the lateral and skew positions of the sheet and feeds the information back to controllers which manipulate the lateral and skew actuator. The current devices, which may use the method of U.S. Patent '211 require the use of expensive sensors to obtain benchmark media registration accuracy. Although lower cost sensors may be used, the lower cost sensors do not exhibit consistent input/output properties.
Therefore, it is desirable to provide a method for calibrating edge sensors often and with a sufficient level of precision. Additionally, use of the method for calibrating edge sensors would allow for the use of low cost sensors capable of providing lateral registration of the sheet with high registration accuracy. Furthermore, there is a desire to use a calibration method with low cost sensors that can deliver better resolution than current registration methods by several orders of magnitude.